For the analysis of large molecules such as DNA, peptides, proteins and other biomolecules, mass spectrometry with MALDI is a standard method. For the most part, time-of-flight mass spectrometers (TOF-MS) are used for this purpose, but ion cyclotron resonance (ICR) spectrometers or Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometers as well as high-frequency quadrupole ion trap mass spectrometers, and hybrid quadrupole time of flight (Q-TOF) mass spectrometers are all applicable for these applications. Normally, biomolecules are in an aqueous solution, but is not uncommon for these important building blocks to be dissolved in solutions that contain varying levels of organic solvents (such as acetonitrile), particularly when reversed phase chromatography is used for isolation and fractionation of complex mixtures of these molecules.
In MALDI mass spectrometry, analyte is mixed with a matrix solution and deposited on a MALDI sample plate for subsequent drying and crystallization. In the drying process, crystal growth of the matrix is induced and analyte molecules become co-crystallized with the matrix. The MALDI sample plate is then inserted into a mass spectrometer and laser beam is directed to the sample plate. Photon bombardment causes the matrix and the analyte to be desorbed and ionized without substantially fragmenting the analyte. The desorbed ions are then mass analyzed in the mass spectrometer. The matrix is an energy absorbing substance which absorbs energy from the laser beam thereby enabling analyte to desorb from the sample plate.
Various methods are known for applying the sample and matrix to a sample plate. The simplest method of these involves a step of pipetting a solution containing analyte and matrix in a droplet onto a metal (e.g., stainless steel) sample support plate. This droplet wets an area on the metal surface, the size of which corresponds approximately to the diameter of the droplet and is dependent on the hydrophobic properties of the metal surface and the characteristics of the droplet. After the solution dries, the sample spot consists of small matrix crystals spread over the formerly wet area, whereby generally there is no uniform coating of the previously wetted area. In aqueous solutions, most of the small crystals of the matrix generally begin to grow at the periphery of the wetted area on the metal plate, growing toward the inside of the wetted area.
It is known that specimens are non-homogeneously distributed on and/or within the lattice that located at the specimen periphery. It is further known that some of these matrix crystals bear more biomolecules than others. Thus, as the laser covers a search area at the specimen periphery, it scans “sweet spots” having a comparatively higher specimen concentration in the matrices.
MALDI-MS performance suffers chiefly from analysis insensitivity. The sample plates that are used in MALDI-MS are typically metallic plates due to the need to apply a voltage across the plate. Stainless steel plates are the most widely used trays because of its chemical stability and proper work function for ionization. However, these give a smooth hydrophilic surface where the applied specimen drop spreads over a relatively large area before drying and forming crystals.
To solve this problem, a stainless steel plate coated with a 30-40 μm thick layer of hydrophobic polytetrafluoroethylene (also known as “PTFE” or Teflon (RTM)) with 200-800 μm diameter hydrophilic spots on it has been provided in U.S. Pat. No. 6,287,872 and U.S. Pat. No. 6,952,011 for focusing sample and matrix.
However, another drawback of metallic plates is that they unfortunately often provide unsuitable results due to unintentional contamination with detergents. Since existing metallic sample plates are also expensive, they are used repeatedly. Washing between each use may contaminate the sample plate used for subsequent analysis.
Therefore it is desirable if there is a sample plate wherein the crystal of sample and matrix are located on the sampling spot.
Also, sample plate which can be used in a disposable type or sample-keeping type is required in considering the cost effectiveness.
A metal substrate coated with a conductive polymer or gold has been suggested as a disposable type sample plate in U.S. Pat. No. 6,952,011 or U.S. Pat. No. 6,825,465.